Find out the base b, which of the other two electrodes is the collector c and which is the emitter e? At this time, we can use the method of measuring the penetration current ICEO to determine the collector c and the emitter e.
(1) For NPN type triode, the measurement circuit of penetration current. According to this principle, use the black and red test pens of the multimeter to measure the forward and reverse resistances Rce and Rec between the two poles upside down. Although the deflection angle of the multimeter pointer is very small in the two measurements, if you observe carefully, there will always be a deflection. If the angle is slightly larger, the current flow must be: black test lead → c pole → b pole → e pole → red test lead, the current flow is exactly the same as the arrow direction in the triode symbol, so the black test lead is connected at this time. It is the collector c, and the red test pen must be connected to the emitter e.
(2) For the PNP type triode, the principle is also similar to the NPN type, and its current flow must be: black test lead → e pole → b pole → c pole → red test lead, and the current flow is also consistent with the direction of the arrow in the triode symbol. Therefore, at this time, the black test lead must be connected to the emitter e, and the red test lead must be connected to the collector c.